Many types of seal materials have been considered for use in high-temperature rechargeable batteries/cells for joining different components. Sodium/sulfur or sodium/metal halide cells generally include several ceramic and metal components. The ceramic components include an electrically insulating alpha-alumina collar and an ion-conductive electrolyte beta-alumina tube, and are generally joined or bonded via a sealing glass. The metal components include a metallic casing, current collector components, and other metallic components which are often joined by welding or thermal compression bonding (TCB). However, metal-to-ceramic bonding can sometimes present some difficulty, mainly due to thermal stress caused by a mismatch in the coefficient of thermal expansion for the ceramic and metal components.
The metal-to ceramic bonding is most critical for the reliability and safety of the cell. Many types of seal materials and sealing processes have been considered for joining metal to ceramic components, including ceramic adhesives, brazing, and sintering. However, most of the seals may not be able to withstand high temperatures and corrosive environments.
A common bonding technique involves multiple steps of metalizing the ceramic component, followed by bonding the metallized ceramic component to the metal component using TCB. The bond strength of such metal-to-ceramic joints is controlled by a wide range of variables, for example, the microstructure of the ceramic component, the metallization of the ceramic component, and various TCB process parameters. In order to ensure good bond strength, the process requires close control of several parameters involved in various process steps. In short, the method is relatively expensive, and complicated, in view of the multiple processing steps, and the difficulty in controlling the processing steps.
Brazing is another potential technique for making the ceramic-to-metal joints. A braze material is heated above its melting point, and distributed between two or more close-fitting parts by capillary action. However, most of the brazing materials (or braze materials) have limitations that prevent them from fulfilling all of the necessary requirements of high temperature batteries. Moreover, some of the commercial braze materials can be quite expensive themselves; and using them efficiently in various processes can also be costly.
It may be desirable to develop new braze alloy compositions that have properties and characteristics that meet performance requirements for high temperature rechargeable batteries, and are less complicated and less expensive to process, as compared to the existing sealing methods.